Satellite-based navigation systems provide position information for a variety of applications. The position information is determined with respect to distances between receivers and transmitters. GNSS (Global Navigational Satellite System) such as Global Positioning System (GPS)/Navstar or GLONASS provide specific examples of satellite-based navigations. In particular, GPS includes a number of geo-synchronous satellites that simultaneously transmit signals. A GPS receiver determines its position by computing the relative times of arrival (TOA) of simultaneous signals. GPS satellites transmit ephemeris data that includes satellite positioning data and timing data. The timing data is used to synchronize the receiver's clock to the clock of the satellite. This allows for the use of less accurate clocks by the receiver. The satellite positioning data includes two positioning components, a code-based component and a carrier-frequency-based component. GPS receivers determine the position of the receiver by comparing locally generated code and/or carrier components using the timing data. The locally generated components are values measured against the received signal from each satellite to determine the signal delay due to the distance from each satellite.
The GPS satellites transmit at two carrier frequencies called L1 and L2. These carrier frequencies carry a pseudorandom (PRN) code or PN sequence that is known by the receivers and that is implemented by spread spectrum modulation of the carrier frequencies. The receiver identifies the transmitting satellite based upon the PN sequence. Satellite signals from several satellites are received, separated and decoded using the code division multiple access (CDMA) data transmission technique.
The PRN codes are sometimes described in terms of chipping or chip rates. The terms chip and chipping are used in place of the term bits. Chipping is generally used to denote instances where the bits are not used to directly convey data (i.e., because the receiver already knows the PRN code).
Generally a receiver must identify, or detect, the satellite's transmission. To do so a receiver performs a correlation process that includes multiplying the received signal by a locally generated code generated from a stored code and local clock. The result is integrated to detect the receipt of a satellite's transmission. This integration step is sometimes referred to as an integrate-and-dump procedure. Adjusting the timing of the locally generated code relative to the received signal provides a mechanism for determining the time delay between the received signal and the local clock through an observation of the integrate and dump output. This determination is also referred to as acquisition of the satellite's signal. After signal acquisition, the timing of the locally generated code is maintained synchronous to the received satellite signal.
The chip rate of civilian codes is slow with respect to the speed of the signal (e.g., at or near the speed of light) and distance between the satellite(s) and receiver. This results in an inaccurate position determination. Accordingly, some GPS receivers also determine the carrier phase of the transmitted signals. GPS receivers accomplish carrier-phase acquisition and tracking by generating a local signal having the expected carrier frequency. This local signal can be generated so as to include Doppler shifts to the carrier frequency. Once this local signal has been synchronized to the received signal, the receiver operates by adjusting (tracking) the carrier-phase to maintain synchronous operation. Such adjustments to the code-phase and carrier-phase of the locally generated signals (for the purpose of tracking the received signals) are sometimes referred to as code wipeoff and carrier wipeoff, respectively.
Using the abovementioned GPS provides typical accuracies of meters in position and nanoseconds in time. These accuracies allow for the use of GPS in critical technology, such as for aviation, where safety-of-life requires strict limits on navigation errors. However, aviation authorities have not certified GPS as a primary means of navigation for landing operations due to reliability concerns. A specific aviation objective is the use of GPS for autonomously controlled landings. Fully-automated GPS landings depend upon high levels of outside assistance and performance validation. An important research area in both civil and military aviation is ensuring GPS accuracy and integrity during final approach and landing. A significant challenge to landing with GPS, particularly for military users, is to reject radio frequency interference, which can jam reception of the GPS signals. Among the aggressive anti-jam technologies is the multi-element antenna array equipped with adaptive beamforming and nullsteering. Antenna arrays that use space-time adaptive processing can improve the signal to interference plus noise ratio (SINR). This spatial and temporal filtering can, however, introduce time-varying biases into the GPS measurements.
A challenge to implementing a GPS adaptive antenna array is the rejection of interference while limiting or mitigating navigation biases. First, strict limits on code-phase and carrier-phase biases have been identified in order to meet accuracy and integrity requirements. Second, there is a need for space-time adaptive antenna arrays in order to meet interference rejection requirements. However, there is a conflict between integrity-driven bias requirements and the requirement to reject interference through use of an adaptive antenna array.
These and other problems have presented challenges to the implementation of satellite navigation systems.